Methods and apparatus of this kind are used in particular in conjunction with painting systems in the form of robotic painting devices which are widely used for instance in the automotive industry to paint body parts. On the basis of customer behavior, production requirements (such as Just In Time) and the ever increasing number of colors, in particular as regards automobiles, such a painting system frequently must be converted to paints of different colors or to paints of varying properties, illustratively up to 80 changes in paint a day being commonplace. Accordingly modern painting facilities are equipped not only with powerful metering and paint-changing means allowing selection from up to 30 different paints, but also with rinsing and cleaning units precluding undesired residues of the previously used paint from reaching the particular workpiece surface after a change of paint has taken place.
Known methods and apparatus of the initially cited species carry out a cleaning stage between two operational stages involving different paints. This cleaning stage substantially applies to treating a feed line—which moves the paint from a selected supply tank of a color changing or paint changing unit to a deposition device (atomizing unit or the like)—by means of a cleaning substance typically in liquid form flowing through said feed line. Said cleaning substance is forced by compressed air through the feed line either in the direction of paint flow (German patent document 20 43 789 C3) or in the opposite direction (German patent document 91 10 650 U1). In both cases the cleaning substance must be removed from the feed line before the next operational stage begins. The cleaning effect may be enhanced by moving to-and-fro a ball or another body—generally called “pig”—in the feed line (European patent document 0 888 825 A2).
A problem arises with such methods and apparatus for cleaning feedlines in that the contemporary liquid paints and especially their hardeners are exceedingly sensitive to oxygen. Even minute paint residues react under ambient atmosphere, forming solid clumps or chunks which, while being small, nevertheless are visible on a smooth, painted surface and render the pertinent workpiece nearly useless. Such clumps being inevitable on account of the compressed air used in cleaning, it must be reliably removed from the feed lines before the new paint reaches the deposition unit, provided that, on one hand, comparatively large quantities of cleaning substance be used. On the other hand, a cleaning substance free of oxygen or air must remain in the feed line during the entire time interval between two operational stages using different paints in order to preclude formation during said time interval even of the tiniest air bubbles and hence paint clumps. As a result, at the beginning of new operational stage, i.e. at the beginning of a new painting stage, not only the cleaning substance per se, but furthermore a given minimum quantity of follow-up paint must be expelled from the deposition device and be transferred into a collecting container before the actual painting may start, in order to reliably preclude any residual quantities of the cleaning substances B which usually contain a solvent B from reaching the workpiece surface to be painted.
The quantities of paint and cleaning substance that are lost in this manner during each cleaning stage are considerable and amount to substantial costs. Also the paint and the cleaning substance must be removed as special wastes, entailing further costs and ecological loads. These factors apply regardless of the cleaning procedure being enhanced using a so-called pig or not.